Check if EMBEDLY_KEY has been set

This will check if EMBEDLY_KEY has been set in settings. If not
it will return a nice error message instead of throwing a 500
error.
Modified embeds.py to make it work

The Embedly API was not working at all because the return
value of Embedly (client) offered the attributes in dictionary
style and not as an object. Now everything seems to be fine.
Explorer interface was cluttered by too many horizontal dividing sections in the header. 'Explorer' title has been dropped in favour of making the current section index page feel more like the title. breadcrumb also toned down
Some icon font use converted to REM due to IE9's incompetent understanding of pseudo-elements
Handle SQLite substr syntax

Addresses https://github.com/torchbox/wagtail/issues/11
Added multi-backend support into wagtail search. Added Database search backend
Merge branch 'master' of github.com:torchbox/wagtail
Search: Updated search handlers and tests to use get_search_backend
Search DB Backend: Don't ignore the fields parameter
Merge remote-tracking branch 'upstream/master'
output template errors on preview into the preview window - fixes #2
Search: Unit tests
Search: Fixed issue with query string normalisation not removing double spaces
More unit tests
Merge branch 'master' of https://github.com/spapas/wagtail into spapas-master
Search: Various tweaks to backend importer
Search: more unit tests
update_index command is now callable from unit tests
DB backend now doesnt attempt to search on callable fields
More improvements to unit tests